1. Department of Algebra and Geometry, Palacký University Olomouc, Faculty of Science, 17. listopadu 12, 771 46 Olomouc, Czech Republic
2. TU Wien, Faculty of Mathematics and Geoinformation, Institute of Discrete Mathematics and Geometry, Wiedner Hauptstraße 8-10, 1040 Vienna, Austria
3. Department of Algebra and Geometry, Palacký University Olomouc, Faculty of Science, 17. listopadu 12, 771 46 Olomouc, Czech Republic